Fri 4 Jun 2004
Celeste Biever, NewScientist.com news service, reports
The first computer network in which communication is secured with quantum cryptography is up and running in Cambridge, Massachusetts.
Chip Elliott, leader of the quantum engineering team at BBN Technologies in Cambridge, sent the first packets of data across the Quantum Net (Qnet) on Thursday. The project is funded by the Pentagon’s Defense Advanced Research Projects Agency.
Currently the network only consists of six servers, but they can be integrated with regular servers and clients on the internet. Qnet’s creators say the implementation of more nodes in banks and credit card companies could make exchanging sensitive data over the internet more secure than it is with current cryptography systems.
The data in Qnet flows through ordinary fibre optic cables and stretches the 10 kilometres from BBN to Harvard University. It is encrypted using keys determined by the exchange of a series of single, polarised photons.
The first money transfer encrypted by quantum keys was performed between two Austrian financial institutions in April 2004. But Qnet is the first network consisting of more than two nodes to use quantum cryptography - a more complex challenge.
“Imagine making a phone call. If you just have one possible receiver, you wouldn’t even need buttons,” explains Elliott. “But with a network you need a system that will connect anyone on the network to anyone else.” In Qnet, software-controlled optical switches made of lithium niobate crystals steer photons down the correct optical fibre.
Intruder detection
Quantum cryptography guarantees secure communications by harnessing the quantum quirks of photons sent between users. Any attempt to intercept the photons will disturb their quantum state and raise the alarm.
But Elliott points out that even quantum cryptography “does not give you 100 per cent security”. Although quantum keys are theoretically impossible to intercept without detection, implementing them in the real world presents hackers with several potential ways to listen in unobserved.
One example is if a laser inadvertently produces more than one photon, which happens occasionally. An eavesdroppper could potentially siphon off the extra photons and decrypt the key, although no one has actually done this.
“However Qnet is more secure than current internet cryptography,” says Elliott, which relies on “one way functions”. These are mathematical operations that are very simple to compute in one direction, but require huge computing power to perform in reverse.
The problem is, according to Elliott, that no one has actually proved that they cannot be solved in reverse. “So who’s to say that someone won’t wake up tomorrow and think of a way to do it?”
Large and expensive
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At the moment computers capable of quantum cryptography are large and expensive, because they are custom-made. Elliott imagines a Qnet-like system may first appear in banks, for whom these factors might be less of a problem.
Another limitation is that, for distances over 50 kilometres, the photon signal is degraded by noise, and it is unclear as yet how this problem will be overcome.
However, quantum keys can potentially be exchanged over much larger distances through the air. Tiny, aligned telescopes can send and detect single photons sent through the air.
The distance record for this form of transmission is currently about 20 kilometres. But calculations suggest that photons transmitted through the air could be detected by a satellite, which would enable data to be sent between continents.